At the present time, there is considerable interest in the adoption of an aspect ratio of 16:9 for television pictures, instead of the existing ratio of 4:3. One particular problem in adopting this new aspect ratio is that it will probably be necessary initially to transmit the 16:9 pictures in a manner compatible with receivers having a 4:3 display so that the same signal gives acceptable pictures on receivers both with 4:3 displays and with 16:9 displays.
One method of overcoming this problem involves presenting the pictures on a 4:3 display in a "full letterbox" format as illustrated in FIG. 1. With this format, the number of active scanning lines containing picture information is 432 lines rather than the normal. 576 lines for a 625-line scanning standard. The main objection to this full letterbox format is the presence of broad black borders at the top and the bottom of the 4:3 display. There is the further disadvantage that vertical detail is lost when the number of active picture lines is reduced.
Another method is to allow the 16:9 picture to fill the whole of the 4:3 display, with no black borders, in which case the picture detail will be stretched vertically by 25%, i.e., people would become tall and thin. This method has the advantage that no vertical detail is lost and it would therefore be sensible to use it in studios where the scan height of 4:3 displays could be adjusted if desired to give the correct geometry. For broadcasting to existing domestic receivers, a 25% vertical stretch would be too noticeable to be acceptable; however, the use of a small amount of vertical stretch could provide a useful compromise between the disadvantages of the full letterbox and full-height formats.
A further method is so-called edge compression in which a central region of each line is uncompressed or is compressed uniformly which a marginal portion o each at the left and right edges of the image display is compressed to change the aspect ratio of the image overall. Two such edge compression methods are described in United Kingdom Patent application 2,191,060 and U.S. Pat. No. 4,605,952.